The invention relates to the field of laminated rotors for electrical machines and particularly to laminated rotors with interior permanent magnets (PM).
Laminated rotors are typically assembled by stacking laminates, such as annular discs or segments of discs, on a cylindrical ring or directly on the rotor shaft. In a conventional laminated rotor, the laminates are all formed of a ferromagnetic material, such as silicon steel. Insulation layers are interleaved between the laminates. The insulation layers are typically each a coating on ferromagnetic laminates.
Permanent magnets are contained in cavities inside the laminated rotor. Magnetic flux from the magnets extends from the surface of the rotor to the stator. The permanent magnets may be inclined with respect to a chord of the rotor, may be parallel to a chord or having another orientation with respect to a cord. The cavities for the magnets extend parallel to the axis of the rotor and pass through the each of the laminates stacked on the rotor. Typically, the cavities extend to near the outer surface of the rotor.
The bridges associated with the cavities and magnets, especially the bridges near the rotor surface, tend to be thin. The bridges are the rotor material between adjacent cavities and between the cavities and the rotor outer surface. The bridges tend to be made as thin as the rotor material will allow in view of the centrifugal forces and stresses applied to the bridges.
Centrifugal forces are created by rotation of the rotor and the mass of the permanent magnets and laminates. The stresses caused by these centrifugal forces are concentrated at the relatively narrow regions of the laminates between the radially outer ends of the cavities and the outer surface of the rotor. These narrow regions between the radially outer ends of the cavities and the outer surface of the rotor are referred to as outer bridges, or sometimes simple bridges.
Other narrow regions in the rotor include the regions between cavities where there are more than one cavity per pole. These regions are referred to as inner bridges and may also experience stress concentration when subjected to centrifugal forces. Further, the inner bridges may also provide paths for magnet flux leakage.
In addition to centrifugal forces, electromagnetic forces may cause stress concentration in the inner and outer bridges. Electromagnetic forces may be especially strong during short circuit fault conditions. The stresses due to electromagnetic forces may be most noticeable, as compared to centrifugal forces, in medium and low speed machines.
The ferromagnetic materials forming the laminates (laminates) tend to have low tensile strengths and are susceptible to failure under high centrifugal loads. The bridges of the conventional ferromagnetic laminates are made relatively thick (wide) to account for the weak tensile strength of the ferromagnetic material and to withstand the high centrifugal forces resulting from the mass of the permanent magnet bars and the high rotational speed of the rotor.
Bridges provide a parallel path for the magnet flux. One part of magnet flux passes through air gap and links with the stator winding. That is “useful” or “working” part of the magnet flux that contributes to the work performed by the rotor and stator. Another part of magnet flux passes through bridges and does not link with the stator winding. The flux passing through the brides is leaked or lost flux that does not contribute to the work accomplished by the rotor and stator.
The magnetic flux leakage from the rotor increases with the thickness of the bridges in the laminates. The flux leakage occurs as flux, which does not cross air gap between stator and rotor, and does not link with an armature winding in the stator. The design of a conventional laminated PM rotor involves balancing the need for sufficient strength in the bridges of the laminates with the need to reduce magnetic flux leakage.